Additive fabrication processes for producing three dimensional articles are known in the field. Additive fabrication processes utilize computer-aided design (CAD) data of an object to build three-dimensional parts layer-by-layer. These three-dimensional parts may be formed from liquid resins, powders, or other materials.
A non-limiting example of an additive fabrication process is stereolithography (SL). Stereolithography is a well-known process for rapidly producing models, prototypes, patterns, and production parts in certain applications. SL uses CAD data of an object wherein the data is transformed into thin cross-sections of a three-dimensional object. The data is loaded into a computer which controls a laser beam that traces the pattern of a cross section through a liquid radiation curable resin composition contained in a vat, solidifying a thin layer of the resin corresponding to the cross section. The solidified layer is recoated with resin and the laser beam traces another cross section to harden another layer of resin on top of the previous layer. The process is repeated layer by layer until the three-dimensional object is completed. When initially formed, the three-dimensional object is, in general, not fully cured and therefore may be subjected to post-curing, if required. An example of an SL process is described in U.S. Pat. No. 4,575,330.
The liquid radiation curable resin used in stereolithography and other additive fabrication processes for forming three-dimensional objects can be solidified by light energy. Typically, liquid radiation curable resins are cured by ultra-violet (UV) light. Such light is typically produced by lasers (as in stereolithography), lamps, or light emitting diodes (LEDs). See PCT Patent Application PCT/US10/60677, filed on Dec. 16, 2010, and incorporated by reference in its entirety. The delivery of energy by a laser in a stereolithography system can be Continuous Wave (CW) or Q-switched pulses. CW lasers provide continuous laser energy and can be used in a high speed scanning process.
With known resins, it is typical that the final color and/or clarity develops in the three dimensional article as it is cured. Known resins may be clear in liquid forms and form opaque three-dimensional articles upon cure. Other known resins may be colorless in liquid form and capable of curing into colored three-dimensional articles. Furthermore, some resins appear as a first color in liquid form and turn a second color upon cure.
Throughout this patent application the term color is defined as follows: color (or colour, alternative spelling) is the visual perceptual property corresponding in humans to the categories called red, yellow, green, etc. Black is the visual perception of absence of all color, whereas white is the visual perception of all colors. Color derives from the spectrum of light (distribution of light energy versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Color categories and physical specifications of color are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra. Typically, only features of the composition of light that are detectable by humans (wavelength spectrum from 400 nm to 700 nm, roughly) are included, thereby objectively relating the psychological phenomenon of color to its physical specification.
Color and transparency are two distinct principles. For instance, something may visually appear perfectly clear and still colored. For instance, certain colored glass is entirely transparent to the eye and possesses a color. Similarly, something may be colorless and also clear or opaque. Colorless is defined as lacking all color. For instance, pure liquid water is clear and colorless. An article that is visually perceived as perfectly clear and as a color, for instance, blue, is reflecting the blue color while allowing all other wavelengths of light to pass through. When a viewer perceives white, the article will appear less transparent because all colors are being reflected back at the viewer and thus not passing through the article.
In recent years, the demand for liquid radiation curable resins that produce three-dimensional articles that have excellent dimensional accuracy, shape stability, mechanical properties, and the like has increased. Along with this development, demand has grown for three-dimensional articles that possess a desired color or transparency/opacity, and also have the mentioned excellent properties. These colored three-dimensional articles are useful because they are aesthetically pleasing, can mimic the appearance of commercial materials, and may possess light-shielding properties. Along with this development, the demand for radiation-curable compositions in which the color or opacity can be selectively controlled during curing has also increased.
Meeting the challenges of producing selectively colored three-dimensional articles is described in U.S. Pat. No. 6,133,336, issued Oct. 17, 2000 and assigned to Zeneca Limited. This patent describes a method of curing and adding color to a three-dimensional article using light at a single wavelength, and at a lower and a higher dose. The lower dose of light is used to cure the liquid resin to form a solid and the higher dose of light is used to add color to the resin. The process claimed is only for adding color, not removing color. This patent also claims a composition for a photocurable and photocolorable resin. However, the disclosed composition has poor mechanical properties and poor color stability. For instance, after initial curing, the uncolored sections of the article become colored over time in ambient light. Such problems are common with photoresponsive coloring techniques.
U.S. Pat. No. 5,677,107, assigned to Spectra Group Limited, Inc., discloses a method for preparing and selectively coloring a three-dimensional article by adding or removing color. The coloring agent is photoresponsive and the method claimed is dependent on using a photoresponsive coloring agent.
U.S. Pat. No. 5,942,554, assigned to Spectra Group Limited, Inc., discloses a method of effecting color change in polymeric bodies of either thermal curable or photocurable resins. The color-changing compound is sensitive to acid produced during polymerization of the resin. The acid is produced from the initiating species which are activated by either light or temperature. The color change occurs when the coloring agent is exposed to the acid.
U.S. Pat. No. 6,664,024, assigned to American Dye Source, Inc., disclose a photocurable resin composition for forming three-dimensional objects that can be selectively colored that utilizes a photoactivated coloring compound.
U.S. Pat. No. 6,649,311, assigned to Vantico Limited, discloses a resin for use in forming three-dimensional objects that can use a photosensitive coloring compound contained in microcapsules. Similarly, U.S. Published Patent Application No. 2004/0076909 discloses a liquid resin composition for use in forming three-dimensional objects which comprises particles dispersed in the composition which are micro-capsules containing a photosensitive color changing composition.
U.S. Published Patent Application No. 2004/0170923, assigned to 3D Systems, Inc., discloses colored resins useful in forming three-dimensional objects; however, such resins cannot be selectively colored by exposure to various doses of light.
U.S. Pat. No. 6,746,814, assigned to the inventor, discloses a method for selectively coloring or shading an article produced by overexposing the liquid resin to radiation during cure and then heating the entire model with an effective amount of heat in order to induce a color change in the overexposed sections of the article. No coloring or transparency modifying agent is used.
It would be desirable to develop a liquid radiation curable resin that can cure into a three-dimensional article wherein the color and/or transparency of the three-dimensional article can be selectively controlled to be substantially clear and/or colorless in some areas and have a desired color and/or transparency in other areas while having excellent mechanical properties. Moreover, it would be desirable to develop a liquid radiation curable resin wherein the color and/or transparency of the selectively controlled sections remain at a substantially constant color or no color, and/or transparency over time.